The long-term objectives of this grant are to improve the use of old antitumor agents and to develop new agents. The specific aims of the current application are: (1) To further determine the relationships between TG incorporation into DNA, DNA damage and cytotoxicity. The primary method to achieve this aim is to measure these parameters in synchronized cells treated with TG. It is proposed that those effects causally related to the mechanisms for toxicity will correlate in a time-and dose- dependent fashion. The nature of the damage in "TG-containing DNA" and in "DNA synthesized from a TG-DNA template" will be carefully determined and correlated with the other parameters. Another approach to achieve this aim will be to compare the TG results with those observed with 6-selenoguanine, a TG analog which has been reported to produce "delayed cytotoxicity" (similar to TG) but not to be incorporated into DNA. Finally, we propose to develop mutant (resistant) cell lines which incorporate TG Into DNA but fail to exhibit cytotoxicity and/or DNA damage as another means to achieve this aim. (2) To determine whether the mechanisms of action observed in animal cells are applicable to human leukemic cells. To achieve this aim, the mechanisms of action of TG and MP will be assessed in human leukemic cells in tissue culture. Specifically, are alterations in purine metabolism related to the mechanisms or is incorporation into nucleic acids involved? (3) If the results obtained suggest a strong correlation between TG Incorporation into DNA, DNA damage and cytotoxicity, the following additional specific aim will be explored: What is the mechanism for the TG-induced DNA damage? The methods to achieve this aim will be dictated by the nature of the DNA damage ascertained above. For example, evaluation of the roles of "repair mechanisms" in the formation of strand breaks will necessarily require a different experimental approach than will an evaluation of the "protein-associated" DNA lesions. The health relatedness of this project is to provide a clearer understanding of how these drugs are effective in curative regimens in some, but not all, children with leukemia.